The p53 protein is an important tumor-suppressor that is negatively regulated by MDM2, itself a p53-induced protein. Here we used nutlin-3A, a reversible inhibitor of MDM2, to activate the p53 response in HCT116 cells and isogenic miRNA-processing defective Drosha and Dicer knockout sublines, to determine the role of miRNAs in this feedback loop. We provide evidence that miRNAs inhibit MDM2 mRNA and protein levels when cells are exposed to nutlin-3A twice, separated by a period of recovery. Surprisingly, MDM2 transcripts with the longest 3'UTRs escaped miRNA-mediated repression, while specific isoforms with shorter 3'UTRs were attenuated in parental cells and over-expressed in the miRNA processing defective knockouts. We propose that miRNA recognition sites are placed in a more accessible context within these shorter 3'UTR variants. Thus, polyadenylation site selection represents a novel regulatory mechanism in the p53 response, with the potential to impact the p53-MDM2 feedback loop and p53-mediated tumor suppression.